1. Field of the Invention
This invention relates to a control system and method for automatic welding and more particularly to a system and method capable of identifying the location and shape of a particular isotherm of the weld pool produced in the welding of dissimilar metals, determining the deviation of the measured isotherm shape and location from a desired shape and location respectively of said isotherm, and adaptively controlling the arc intensity and direction to reduce the deviations of the shape and location of actual weld pool isotherm patterns from desired isotherm patterns.
2. Background Information
During the welding of various dissimilar metal combinations, substantial arc deflection toward one base metal causes significant dilution of that base metal into the weld pool resulting in composition gradients, differing solidification characteristics, and incomplete fusion along the root of the weld joint. Similar poor results can result in welding of seams that are metallurgically similar but which have dissimilar geometry such as thickness, width and cross section. In hand welding, the shape and location of the pool is determined by observation of the operator and pool location and shape are compensated by manually adjusting the location of the electrode with respect to the seam being welded. Unfortunately, hand welding is a very tedious art and not generally susceptible to repetition for long periods of time. Many mechanical and electronic devices have been employed in the past to automate the welding process, these devices being generally directed to tracking the seam to be welded or the arc which emits from the electrode. Because of the various circumstances which exist at the weld interface, the arc may stray or drift.
There are many known causes of arc stray or drift. Among these are: welding of dissimilar metals; welding metals of similar metallurgical structure but varying in conduction characteristics resulting from differences in size and shape; convection currents resulting from the movement of shielding gas at the weld interface; localized magnetic effects; and, unequal grounding between the two pieces being welded, the latter being especially true at the beginning of the weld.
Various methods have been employed for sensing the location of the weld and various methods have been employed for re-directing the arc in attempts to equalize the impingement of the arc upon the metals to be welded. For example, in U.S. Pat. No. 2,994,763 to Schultz, a pair of sensors for sensing the radiation of the arc were located one on either side of the arc. Deviations were determined and the position of the arc was adjusted magnetically to follow a given path. This arrangement, however, relied on secondary information to determine the location of the weld pool. That is, if the arc intensities on both sides of the weld seam were equal, then it was presumed that the weld pool was correctly formed and in the right location. This assumption is subject to considerable error, especially in the dissimilar metals situation.
U.S. Pat. No. 3,627,972 to Iceland, et al. introduced a concept of looking at the total radiant energy emitted from the weld pool to control an overall average size of the weld pool. Iceland was premised on the theory that an undetectable temperature gradient existed in the weld pool due to thermal inertia and boiling of the weld pool and that the size of the weld pool and not its temperature changed.
Attempts have been made to control the quality of the weld by controlling the dwell time of the arc alternately on the two work pieces. In U.S. Pat. No. 3,551,637 to Lampson, spot thermal sensors were described as being deployed on either side of the weld seam and a magnetic deflection apparatus was employed for systematically deflecting the arc from one work piece to another.
More recently, systems for determining the centroid of a weld pool and readjusting the position of the center of the weld pool have emerged. U.S. Pat. No. 4,594,497 to Takahashi discloses one method of sensing of an isothermal pattern and adaptive control of the process in relation to isothermal lines which are sensed by an infrared camera. The method of Takahashi senses an isothermal pattern and finds the centroid thereof. Based on information obtained, the control system repositions the welding torch to relocate the centroid.